Internal combustion engines may use variable cam timing (VCT) to improve fuel economy and emissions performance of a vehicle. The VCT device may include a vane type cam phaser that is controlled by an electromechanically actuated spool valve. The spool valve may direct flow of a hydraulic fluid, such as oil, from one side of the vane to the other, such as from a retard side to an advance side. The VCT device may include more than one oil circuit connecting one side of the vane to the other through which the flow of a hydraulic fluid may be directed. The phaser may be oil pressure actuated, wherein the actuation of the phaser is dependent on oil pressure in the circuit. Alternatively, the phaser may be cam torque actuated wherein the actuation of the phaser is dependent on torque generated during cam actuation.
One example of a cam torque actuated VCT phaser is shown by Smith et al. in U.S. Pat. No. 8,356,583. Therein, the VCT device is configured with a hydraulically activated locking pin in an intermediate position (herein also referred to as a mid-lock position). Conventional VCT devices may include a locking pin at one end of the range of the phaser. The VCT device of Smith also utilizes two independent oil circuits, herein referred to as the phasing circuit and the detent circuit. In the mid-lock VCT phaser of Smith, a piloted valve is included in the phaser's rotor assembly and is moveable from a first position to a second position. When the piloted valve is in the first position, hydraulic fluid is blocked from flowing through the piloted valve. When the piloted valve is in the second position, hydraulic fluid is allowed to flow between a detent line from the advance chamber and a detent line from the retard chamber through the piloted valve and a common line, such that the rotor assembly is moved to and held in the intermediate phase angle position relative to the housing assembly. Detent lines communicating with the advance chamber or retard chamber are blocked when the VCT phaser is at or near the intermediate position. The spool valve has three regions of operation, namely Auto-Lock, Retard, and Advance in the specified order. The auto-lock region may hereupon be referred to as the detent region. Specifically, when the spool valve is commanded to the retard or advance regions, the piloted valve is in the first position, and fluid is blocked from flowing through the detent circuit lines. Additionally, fluid may flow from one side of the vane to the other via the phasing circuit lines. When the spool valve is commanded to the detent region, the piloted valve is in the second position, and fluid is free to flow from the advanced or retarded chamber, through the detent lines and the piloted valve, and into the opposite chamber through a common fluid line. Additionally, fluid is blocked from flowing through the phasing circuit lines.
However, the inventors herein have identified potential issues with such a VCT system. In the case of a cam torque actuated (CTA) mid-lock VCT phaser, when the spool valve is commanded to move into the retard or advance region, the detent circuit is prevented from “auto-locking” the cam phaser by the presence of high pressure oil going into the piloted valve. If either the spool valve is commanded to move in the retard, null, or advance region, and insufficient hydraulic pressure is present, or if the spool valve is commanded to the region between the detent and retard regions, the detent oil circuit may engage and compete with the phasing circuit for hydraulic control of the cam phaser position. In one example, insufficient pressure may occur due to leakage through hardware components of the detent circuit. As a result, the cam phaser may end up in the mid-lock position with the locking pin engaged when a command to retard the cam phaser position was intended. In another scenario, the phaser may not predictably respond to spool valve commands due to additional and erratic actuation via the flow of fluid through detent circuit lines. Further still, the cam phaser may be retarding when a command to auto-lock the cam phaser was intended. Any of these scenarios may result in engine performance degradation.
In one example, the issues described above may be addressed by a method, comprising indicating degradation of a variable cam timing phaser based on cam torque oscillations being higher than a threshold, the cam torque oscillations learned during a condition while a spool valve of the variable cam timing phaser is outside a no-fly zone. In this way, situations in which both the detent circuit and the phasing circuit are engaged may be detected and indicated in a timely manner, allowing mitigating actions to be taken.
As an example, average cam torsion magnitudes on each cam tooth may be mapped as a function of engine speed. The mapping may be performed during selected engine operating conditions. Additionally, the map may be updated with current cam torsion measurements during steady-state engine speed conditions. Subsequently, during phaser operation, cam torsion magnitudes may be monitored to determine whether both the detent and phasing circuits are engaged. Specifically, if the estimated cam torsion values exceed the mapped values at a given engine speed by more than a threshold factor, the engine may be identified as operating with both the detent and phasing circuits engaged. Additionally, mitigating actions may be taken to remove the engine from this condition and reduce unpredictable control of the cam phaser. For instance, the engine controller may command the spool valve to the auto-lock region, thus averting competition for cam phaser control from the phasing circuit while the detent circuit is inadvertently engaged. Additionally, operation within the overlap region may be indicated to the rest of the control systems.
In this way, simultaneous activity in both the detent circuit and phasing circuit may be quickly detected. Additionally, mitigating steps may be taken to prevent unpredictable phaser control. For instance, the spool valve may be commanded to the auto-lock region to prevent activity in the phasing circuit, which may compete with activity in the detent circuit for control of cam phaser position. In another instance, if the detent circuit and phasing circuit were simultaneously engaged due to operation of the spool valve in the overlap region, indicating operation within the overlap region may prompt adaptive learning of the boundaries of the overlap region in an effort to prevent further commands to operate the spool valve in this region.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.